President Reagan said today he hoped the latest Soviet proposal at the Geneva arms talks meant the superpowers have "reached a turning point" in building a "safer and more peaceful world," and officials promised a U.S. response by the time negotiations resume in mid-September.

While the latest round did not produce major results, Reagan said he hopes the recent Soviet offers "signal the beginning of a serious Soviet effort" to reduce nuclear arsenals.

Reagan's statement, which echoed language of a conciliatory address he delivered recently in Glassboro, N.J., and similar remarks made today by other U.S. officials, was timed to note the end of the fifth round of nuclear and space talks in Geneva.

Max M. Kampelman, the chief U.S. negotiator, said in Geneva that although there were still "very real and substantive differences . . . at least in some areas we may now have fresh opportunities for serious and constructive discussion."

The chief Soviet negotiator at the Geneva talks, however, sharply criticized the United States for blocking progress by failing to present any new proposals. Viktor Karpov declared in a statement that assessments such as Kampelman's "do not reflect the actual state of negotiations and present it in a distorted light."

Interjecting a more restrained note into today's positive talk, one U.S. hard-line official involved in policy-making said that those who say the Soviets have at last gotten down to brass tacks have to realize, "If we're not careful, we'll have to sit on them."

In late May, the Soviets introduced modifications of their earlier proposals in the strategic arms and space defense talks. The new offer linked continued U.S. adherence to the 1972 Anti-Ballistic Missile (ABM) Treaty for 15 years to reductions in strategic forces. In making this proposal, the Soviets modified provisions that Washington had criticized in their past offer; one such modification was the elimination of the American tactical nuclear forces in Europe from the U.S. strategic forces category.

In background talks here and in Washington, U.S. officials today gave both positive and negative views of the Soviet offer.

Illustrating the internal debate within the administration was Edward L. Rowny, special adviser to the president on arms control and widely considered to be a hard-liner.

In a prepared statement, Rowny said the new Soviet proposals caused the latest negotiating round to end on "a fairly positive note." In answer to questions at a breakfast session with reporters, however, Rowny said that "whether what was said by the Soviets was a basis for a U.S. counterproposal had yet to be decided."

One flaw Rowny cited is the Soviet determination to link progress in space weapons talks with movement in the strategic talks, a point of disagreement that Reagan also emphasized.

Rowny said that the Soviet proposals "may open up dialogue for agreement in certain areas," but later said that was "largely on strategic weapons."

In the space defense area, Rowny said research was "nonnegotiable," but added that talks had begun on the "interpretations" of testing and development under the ABM Treaty. He also repeated Reagan's assertion that deployment of any system that may emerge from the Strategic Defense Initiative was negotiable.

However, Rowny said the Soviet proposal to extend U.S. adherence to the treaty for some specific amount of time was not acceptable.

White House spokesman Larry Speakes said today that Reagan is preparing a response to a letter received this week from Soviet leader Mikhail Gorbachev and that U.S. officials are also analyzing the Soviet arms control proposal. But Speakes said he could not say whether the U.S. arms response would be made before the Geneva negotiations resume Sept. 18.

Speakes also said U.S. officials remain hopeful that Reagan and Gorbachev can meet this year in the United States but that no agreement has been reached on a planning meeting between Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze.

Staff writer Walter Pincus in Washington contributed to this report.